1. Field of the Invention
This invention relates generally to the silk screen printing art and more particularly to an improved registration method and apparatus for registering, that is aligning, the screen image(s) to be printed relative to the workpieces to be imprinted with such screen image(s).
2. Discussion of the Prior Art
The printing procedure, commonly referred to as silk screen printing, is very old and well known and utilized to imprint both monochromatic and multicolor images on a vast assortment of articles. A wide variety of screen printing devices have been devised ranging from a simple frame for holding a printing screen, a support for the work piece to be imprinted, and a manual squeegee for wiping a printing ink across the screen to automatic multistation screen printing machines for high speed printing multicolor images on workpieces.
Following is a list of patents disclosing screen printers and related registration systems:
U.S. Pat. No. 3,943,851 dated Mar. 16, 1976, to Inada discloses a system for aligning a sheet containing an image to be printed relative to a printing screen on which the image is to be reproduced.
U.S. Pat. No. 4,463,673 dated Aug. 7, 1984, to Moore discloses a system for aligning a first sheet relative to a second sheet containing an image to be printed on the first sheet.
U.S. Pat. No. 4,669,378 dated Jun. 2, 1987, to Lee discloses a multiple station carousel screen printer including alignment gates at the printing stations for angularly aligning printing screens at the stations with platens supporting workpieces to be printed.
U.S. Pat. No. 4,846,058 dated Jul. 11, 1989, to Farris discloses a system for screen printing a sequence of accurately registered images involving imprinting test images on a transparent register plate to check image registration.
U.S. Pat. No. 4,949,635 dated Aug. 21, 1990, to Padula discloses a multicolor carousel screen printer having screen frame supports which are adjustable to register the printing screens relative to the workpieces to be imprinted.
U.S. Pat. No. 4,993,166 dated Feb. 19, 1991, to Bradley discloses means for adjusting a printing screen frame relative to a screen printer pin bar.
U.S. Pat. No. 5,127,321 dated Jul. 7, 1992, to Proffer discloses a system for pre-registering a set of related film positives relative to one another and a carousel screen printer in which each screen holder and each print head have coacting means for aligning the holder and print head about the rotation axis of the printer turret.
U.S. Pat. No. 5,188,026 dated Feb. 23, 1993, to Fuqua et al and U.S. Pat. No. 5,226,362 dated Jul. 13, 1993, to Iaccino et al disclose multistation carousel screen printers having a calibration pallet rotatable to the print stations in succession, and means at each station for indirectly pin registering a printing screen at each station to the calibration pallet by first pin-registering a pair of alignment brackets at the respective station to the calibration pallet and thereafter pin-registering a screen frame mounting the printing screen to the alignment brackets.
U.S. Pat. No. 5,239,923 dated Aug. 31, 1993, to Belcher et al discloses a multistation screen printer having ball and socket means at each station for effecting screen/pallet registration.
The present invention is concerned primarily with, and will be described in the context of a multistation, multicolor printing machine, specifically a so-called carousel screen printer. It will become evident as the description proceeds, however, that the registration means of the invention may be used on other types of screen printers.
Simply stated, a carousel screen printer comprises a number of print stations spaced circumferentially about a vertical axis, a rotary turret mounting an equal number of work supports, commonly called pallets, also spaced circumferentially about the axis, and means for rotating the turret to align the pallets with the print stations in succession. Each pallet is elevated to and lowered from a raised printing position at each station. Each print station includes a holder supporting a screen frame mounting a printing screen having blocked and unblocked screen apertures or pores defining an image to be printed. Each print station also includes a so-called flood bar movable back and forth across the upper side of the screen for spreading a printing ink across the screen and forcing the ink through the open screen pores. The images on the several printing screens define different portions or color separations of a completed multicolor image to be printed.
Operation of a carousel screen printer involves (a) mounting a workpiece to be imprinted on each work pallet, (b) rotating the printer turret stepwise through its successive printing positions to rotate the work pallets to the print stations in succession, (c) elevating the pallets at the successive print stations to their printing positions in which the work pieces on the pallets contact the undersides of the respective printing screens, and (d) driving the flood bars back and forth across the upper sides of their respective screens while the pallets are in their elevated printing positions to imprint the respective screen images on the work pieces. The several screen images are thereby imprinted in succession on each work piece. Each of these several screen images is printed in a different single color. The shapes of the several screen images and the different colors in which these images are successively imprinted on each work piece are selected to produce on each workpiece a desired composite multicolor image.
Successful screen printing of such a composite multicolor image requires very precise screen image-to-work piece registration in order to achieve the precise registration or alignment of the several color separation images successively imprinted on each work piece. A variety of registration systems for this purpose have been devised. These prior registration systems range from those in which registration of the successive printed images is accomplished by visual observation of the printed images and manual adjustment of each frame as necessary to achieve precise image registration, as in the Farris U.S. Pat. No. 4,846,058, to those registration systems which involve positive mechanical registration, as in the Fuqua U.S. Pat. No. 5,188,026 and Iaccino et al U.S. Pat. No. 5,226,362. The prior visual/manual registration procedures are very time consuming and require the services of a skilled technician and hence are quite costly. The Fuqua and Iaccino et al mechanical registration systems are two stage registration systems which involve a two step registration procedure at each print station, as discussed below, and hence are also relatively time consuming and costly.
The precise image registration essential to multicolor screen printing involves three basic requirements. These requirements are: (1) precise registration (i.e. precise predetermined location or alignment) of each screen image relative to its respective screen frame, (2) precise registration of each screen frame relative to each work pallet, and (3) precise registration of each work piece relative to its work pallet, that is proper placement of each work piece of its work pallet in precise alignment with the pallet.
In the Fuqua and Iaccino et al screen printers, these three requirements are accomplished by utilization of a vacuum table having upstanding registration pins, a calibration pallet having registration holes which replaces one work pallet of the printer during the registration procedure and its rotated to the printer print stations in succession, a screen frame at each print station having registration holes, and screen frame holders at each print station mounting adjustable alignment brackets having registration holes. Image registration involves the following registration procedure.
A film positive transparency bearing a color separation image to be printed at each print station is prepared and secured to the vacuum table with the film positive image located in a predetermined position relative to the registration pins on the table. A screen frame mounting a photo-resist-coated printing screen is placed on the vacuum table over the film positive with the table registration pins extending through the frame registration holes to locate the frame, and hence the frame registration holes, in a predetermined registered position relative to the film positive image. The film positive is then secured to the frame, after which the frame and film positive are removed from the table, exposed, and processed to produce on the screen a negative image corresponding to the film positive image and registered with, that is located in a predetermined registered position relative to, the frame. This part of the registration procedure is repeated for each screen frame of the printer and provides a plurality of screen frames bearing registered color separation screen images to be sequentially imprinted on each work piece.
The remainder of the Fuqua/Iaccino registration procedure involves indirect registration of the screen frame at each print station with the calibration pallet. This is accomplished by initially aligning and then inserting registration pins through the registration holes in the adjustable print station alignment brackets and the registration holes in the calibration pallet to pin-register the alignment brackets with the calibration pallet. The alignment brackets are then fixed in their registered positions. The final step of the Fuqua/Iaccino registration procedure involves aligning and then inserting the registration pins through the registration holes in the respective screen frame and the fixed alignment brackets to pin-register the frame with the alignment brackets.
This latter part of the registration procedure is repeated at each print station. The calibration pallet is then replaced by a normal work pallet, the work pieces to be printed are fixed in the proper registered positions on the several work pallets, and the screen printer is operated to print the successive color separation images on the workpieces. Since the calibration pallet and all the printer work pallets occupy the same, though circumferentially spaced, positions on the printer turret, the Fuqua/Iaccino registration procedure is stated to be effective to register work screen frame image with each work pallet and hence with the work piece on the pallet.